Heating bath for hardening of steel, especially tool steel



Patented Oct. 8, 1935 A UNITED STATES PATENT OFFICE HEATING BATH FORHARDENING OF STEEL,

many, assignors to Richard Zwcignicderlassung der Firma Weber & 00.,Louis Soeat &

Co. in. b. n, Dusseldorf-Reisholz, Germany, a corporation of Germany NoDrawing. Application April 8, 1933, Serial No. 665,082. In Germany April14, 1932 15 Claims.

This invention relates to a heating bath for the hardening of steel,especially tool steel by heating in molten liquid baths at hightemperatures preferably exceeding 1009" C., with subsequent quenchingand, if necessary, tempering.

It is known that in the hardening, for example of tool and high speedtool steels from the molten liquid salt baths, soft surface layers oftenform at high temperatures and following quenching and tempering, whichlayers are subject to rapid wear and are often called soft skin".

It has already been proposed to add ferro-silicon to the hardening bathin order to prevent this soft skin which might very detrimentally affectl the utility of the finished article. By this means a hard surface iscertainly obtained but the decarbonization cannot reliably be prevented,so that the decarbonized layer again becomes soft during tempering orthe use of the tool.

\ It has also been proposed to quench in a carbonizing salt bath thetools heated to a high temperature in ordinary barium chloride baths.The object was, by reintroduction of carbon to return its formerhardness to the soft layer produced during the hardening in the bariumchloride bath and which is due chiefly to decarbonization. Even if thedecarbonizationis undone, the char-' acteristic high speed steelstructure cannot be reestablished in this manner.

Seeing that it has been ascertained that quite pure barium chloridebaths effect no or only very slight decarbonization, it has beenproposed to entirely or almost entirely renew the barium chloride bathsby fresh salt after they have been .used a few times. This method. is,however, un-

economical owing'to the high consumption of salt and is furtheraccompanied by technical dif= ficulties.

The formation of soft skin is avoided according to the invention byemploying a heating bath composed of neutral salts or salt mixtures,such as barium chloride, mixtures of barium chloride with alkalinechlorides and the like, allowing a heating to high temperatures, such as700 to 1500 C. and more, and of small quantities of metallic oxides,such as magnesium oxide, aluminium oxide, beryllium oxide, zirconiumoxide, cerium oxide or mixtures of such oxides, and also smallquantities of boron compounds, especially borax or other substances,such as phosphorous compounds especially pyrophosphates, acting in thesame sense, and especially favouring the dissolving of the metallicoxides in the molten mass and the action thereof on the surface of thematerial to be hardened. Substances, which are suitable for forming theoxides in question, for example magnesium carbonate, chloride obtainedfrom cerite, and the like can be employed instead of the metalliccompounds, such as magnesium oxide and the like. 5

The quantity of the metallic oxides to be added to the bath-dependschiefly upon the working conditions, such as bath composition, treatingtemperature, quantity of working charge and the like. The addition ofmetallic oxides may be for example about 1 to 10%, preferably about 2 wto 7%. e

The percentage of the substances, such as boron compounds, assisting thedissolving and, in conjunction with the metallic oxides, preventing theformation of soft skins, depends chiefly upon the quantity of addedmetallic oxides and the composition of the neutral salt mixture; itgenerally does not amount to more than and preferably not more thanabout 12%.

The effect ofthe metallic oxides is probably chiefly due to the factthat they prevent the decomposition of the neutral salts, for example ofthe barium chloride, whereas the substances to be added, such as borax,serve, besides for assisting dissolving, also for counteractingdecarbonization and thus, together with the metallic oxides, prevent theformation of a soft skin on the hardened material even in the case ofsubsequent tempering treatment. The hardening is generally carried outat bath temperatures between 730 and 1400 C.; the hardening of toolsteels generally takes place at temperatures between 730 and 850 C. andthe hardening of high-speed tool steels at temperatures between 1000 and1400 (3., preferably at about 1300 C.

The invention is carried out for example in the following manner: Thearticles of high-speed tool steel are subjected to a short heattreatment at 40 a temperature of about l000-1350 C., the duration ofwhich depends on the size of the articles, in a bath consisting of 85parts by weight of barium chloride and containing, besides 2.5 to 7parts of magnesium oxide, about 9 to 12 parts of crystalline borax,whereupon they are quenched in known manner and then tempered. Thetempering temperatures are determined by the composition of the steel.As, however, contrary to the known processes, complicated workpieces canbe hardened, without danger of decarbonization, at the maximumtemperature most suitable for the steel, in the case of high speed toolsteel for example at 1300" C., the tempering temperature canconsequently be also very much increased; in.'66

the'caseof high speed tool steel, for example to 550 to 580 C.,and'more.

For producing the bath barium chloride preferably comes into question as'neutral' salt for hardening high speed tool steels and for tool steelchiefly sodium chloride and potassium chloride, if

necessary mixed with barium chloride.

The following baths are, for example, suitable for carrying out theprocess according to the invention:--

Parts by weight (1) Barium chloride '71 Sodium chloride 20 Borax 8Magnesium oxide 1 Parts by weight (2) Barium chloridefree from water88.3 Magnesium oxide (burned and ground) 6.7 Borax (crystalline) 9perfect hardness, whilst reliably avoiding the formation of soft skins.

We claim:-

1. A heating bath for hardening steels which comprises an alkaline earthchloride, as a major portion thereof, a small quantity of a metallicoxide of the group consisting of beryllium oxide, magnesium oxide,aluminum oxide, cerium oxide, and zirconium oxide, and a small quantityof an oxygen-containing compound of the group consisting of anoxygen-containing boron compound and a pyrophosphate, saidoxygen-containing compound when fused being capable of dissolvingmetallic oxides.

2. A heating bath for hardening steels which comprises a mixture of analkaline earth chloride, asa major portion thereof, and a chloride of analkali metal, a small quantity of a metallic oxide of the groupconsisting of beryllium oxide, magnesium oxide, aluminum oxide, ceriumoxide and zirconium oxide, and a small quantity of an oxygen-containingcompound of the group consisting of an oxygen-containing boron compoundand a pyrophosphate, said oxygen-containing compound when fused beingcapable of dissolving metallic oxides.

3. A heating bath for hardening steels which comprises barium chloride,as a major portion thereof, a small quantity of a metallic oxide of thegroup consisting of beryllium oxide, magnesium oxide, aluminum oxide,cerium oxide, and zirconium oxide, and a small quantity of anoxygencontaining compound of the group consisting of anoxygen-containing boron compound and a pyrophosphate, saidoxygen-containing compound when fused being capable of dissolvingmetallic oxides.

4. A heating bath for hardening steels which comprises barium chloride,as a major portion thereof, and a chloride of an alkali metal, a smallquantity of a metallic oxide of the group consisting of berylliumoxide,magnesium oxide, alumi num oxide, cerium oxide and zirconium oxide, anda small quantity of an oxygen-containing compound of the groupconsisting of an oxygen-containing boron-compound and a pyrophosphate,

said oxygen-containing compound when fused 5 being capable of dissolvingmetallic oxides.

5. A heating-bath for hardening steels which comprises an alkaline earthchloride, as a major portion thereof, not more than about 10% of ametallic oxide of the group consisting of beryl- 10 portion thereof, notmore than about 12% or an 20 oxygen-containing compound of the groupconsisting of beryllium oxide, magnesium oxide, aluminum oxide, ceriumoxide and zirconium oxide, and a small quantity of an oxygen-containingcompound of the group consisting of an 25 oxygen-containing boroncompound and a pyrophosphate, said oxygen-containing compound when fusedbeing capable of dissolving metallic oxides.

7. A heating bath for lhardening steels which comprises an alkalineearth chloride, as a major portion thereof, not more than about 10% ofa. metallic oxide of the group consisting of beryllium oxide, magnesiumoxide, aluminum oxide, cerium oxide and zirconium oxide, and not more 35than about 12% of an oxygen-containing compound of the group consistingof an oxygen-containing boron compound and a pyrophosphate, saidoxygen-containing compound when fused being capable of dissolvinmetallic oxides.

8. A heating bath for hardening steels which comprises an alkaline earthchloride, as a major portion thereof, a small quantity of magnesiumoxide and a small quantity of an oxygencontaining compound of the groupconsisting of 5 an oxygen-containing boron compound and a pyrophosphate,said oxygen-containing compound when fused being capable of dissolvingmetallic oxides.

9. A heating bath for hardening steels which 50 comprises an alkalineearth chloride, as a major portion thereof, a small quantity of ametallic oxide of the group'consisting of beryllium oxide, magnesiumoxide, aluminum oxide, cerium oxide and zirconium oxide and a smallquantity of borax.

10. A process for hardening steels by heating in molten baths whichcomprises establishing a molten bath of an alkaline earth chloride, as amajor portion thereof, a small quantity of a metallic oxide of the groupconsisting of beryllium oxide, magnesium oxide, aluminum oxide, ceriumoxide and zirconium oxide, and a small quantity of an oxygen-containingcompound of the group consisting of an oxygen-containing boron compoundand a pyrophosphate, said oxygen-containing compound when fused-beingcapable of dissolving metallic oxides, maintaining a temperature betweenabout 700 C. and about 1500 C. in said bath, immersing a steel articlein said bath to heat the same, and subsequently quenching the heatedsteel article.

11. A process for hardening steels by heating in molten baths whichcomprises establishing a molten bath of an alkaline earth chloride, as a7 I 2,018,821 major portion thereof, and a; chloride of anzirconiumoxide, and a small quantity of an oxygen-containing compound ofthe group consisting of an oxygen-containing boron compound and apyrophosphate, said oxygen-containing compound when fused being capableof dissolving'metallic oxides; maintaining a temperature between about700 C. and about 1500" C. in said bath, immersing a steel article insaid bath to heat the same, and subsequently quenching the in molten.baths which comprises establishing a molten bath of barium chloride, asa major portion thereof, a small quantityof a metallicoxide of the groupconsisting of beryllium oxide, magnesium oxide, aluminum oxide, ceriumoxide and zirconium oxide, and a. small quantity of an oxygen-containingcompound of the group consisting of an oxygen containing boron compoundand a pyrophosphate, said oxygen-containing compound when fusedbeing'capable of dissolving metallic oxides, maintaining a temperature abetween about 700 C. and about 1500 C. in" said bath, immersing a steelarticle in said bath to heat the same-,and subsequently'quen'ching theheated steel article. v

13. A process for hardening steels by heating 1,

in molten baths'which comprises establishing a molten bath ofbarium-chloride, as a major portion thereof, and chloride of an alkalimetal, a Small quantity of a metallic oxide of the group consisting ofberyllium oxide, magnesium oxide, aluminum oxide, cerium oxide andzirconium oxide, and a small quantity of an oxygencontaining compound ofthe group consisting of 'an oxygen-containing boron compound and apyrophosphate, said oxygen-containing combath, immersing heat the sameand subsequently quenching the oxides,

pound whenfused being capable of dissolving metallic oxides, maintaininga temperature between about 700 C. and about 1500 C. in said a steelarticle in said bath to heated steel article. t

-14. A process for hardening tool steels which comprises establishing amolten bath of an alkaline earth chloride, as a major portion thereof, asmall quantity of a metallic oxide of the group 10 consisting ofberyllium oxide, magnesium oxide, aluminum oxide, cerium oxide andzirconium ox. ide, and a small quantity of an oxygen-containing compoundof the group consisting of an oxygen-containing boron compound and apyrophosphate, said oxygen-containing compound when fused being capableof dissolving metallic maintaining a temperature between about 730 C. toabout 850 C. in said bath, immersing a steel article in said bath toheat the same, and subsequently quenching the heated steel article.

15. A process for hardening high speed tool steels by heating in moltenbaths which coniprises establishing a molten bath of an alkaline 25earth chloride, as a major portion thereof, a small quantity of ametallic oxide of the group consisting of beryllium oxide, magnesiumoxide,

